1. Field of Invention
The invention relates to an exhaust gas control apparatus and an exhaust gas control method for an internal combustion engine, which is provided with a sulfur concentration sensor that detects a sulfur constituent of exhaust gas.
2. Description of Related Art
A publication of Japanese Patent Application Laid-Open No. JP-A-2001-303937 discloses an exhaust gas control apparatus for an internal combustion engine, which detects a sulfur concentration of exhaust gas by a sulfur oxides (SOx) sensor that is disposed downstream of an occlusion reduction type NOx catalyst. Publications of Japanese Patent Application Laid-Open Nos. JP-A-6-173652 and JP-A-2000-230419 further disclose related art of the invention.
The structure of the generally employed exhaust gas control apparatus as described above may fail to accurately detect the concentration of sulfur contained in the exhaust gas discharged from the internal combustion engine. When an air/fuel ratio of the exhaust gas is lean, the sulfur constituent such as SOx is oxidized, which will be held in the exhaust catalyst such as the occlusion reduction type NOx catalyst in the form of a sulfate. Meanwhile when the air/fuel ratio of the exhaust gas is rich, the sulfur constituent passes through the exhaust catalyst. Most part of the sulfur constituent that has passed through the exhaust catalyst, however, may be reduced into a hydrogen sulfide (H2S) which is difficult to be detected by the SOx sensor.